C5+ hydrocarbon, such as aromatic hydrocarbon and/or C5+ oligomer of light hydrocarbon, is frequently used as blending components for transportation fuels. In addition to this use, aromatic hydrocarbon is also used for producing petrochemicals such as styrene, phenol, nylon and polyurethanes and many others. C5+ hydrocarbon can be produced by cracking a light hydrocarbon stream such as ethane in the presence of steam (steam cracking). Exposing the combined ethane-steam feed to steam cracking conditions produces a product comprising molecular hydrogen, C4− olefin, other C4− hydrocarbon, and C5+ hydrocarbon, such as C5+ oligomer. The yield of aromatic hydrocarbon and C5+ oligomer from steam cracking is generally much less than the yield of light hydrocarbon. Consequently, complex processes typically are needed for separating and recovering aromatic hydrocarbon and C5+ oligomer from steam cracker effluent. Catalytic naphtha reforming produces a product having a much greater content of aromatic hydrocarbon than steam cracker effluent, but the naphtha feed is itself useful for other purposes such as a motor gasoline blendstock.
Various attempts have been made to overcome these difficulties, and provide an efficient process for producing C5+ hydrocarbon at high yield from a relatively inexpensive feed. For example, processes have been developed for producing light aromatic hydrocarbon (e.g., benzene, toluene, and xylenes-“BTX”) from paraffinic C4− feeds. The processes typically utilize an acidic molecular sieve such as ZSM-5 and at least one metal having dehydrogenation functionality, such as one or more of Pt, Ga, Zn, and Mo. These conventional processes typically operate at high temperature and low pressure. Although these conditions are desirable for producing aromatic hydrocarbon, they also lead to undue catalyst deactivation as a result of increased catalyst coking. Catalyst coking generally worsens under conditions which increase feed conversion, leading to additional operating difficulties.
One way to lessen the amount of catalyst coking is disclosed in U.S. Pat. No. 5,026,937. The reference discloses removing C2+ hydrocarbon from the feed in order to increase the feed's methane concentration. Since ethane, propane, and butanes are less refractory, removing these compounds from the feed decreases the amount of over-cracking, and lessens the accumulation of catalyst coke. The process utilizes a catalyst comprising molecular sieve, an amorphous phosphorous-modified alumina, and at least one dehydrogenation metal selected from Ga, Pt, Rh, Ru, and Ir. The catalyst contains ≤0.1 w t. % of Ni, Fe, Co, Group VIb metals, and Group VIIb metals. The reference also discloses increasing aromatic hydrocarbon yield by removing hydrogen from the reaction, e.g., by combusting the hydrogen with oxygen in the presence of an oxidation catalyst that has greater selectivity for hydrogen combustion over methane combustion.
Processes have also been developed for converting less-refractory paraffinic hydrocarbon to aromatic hydrocarbon with decreased selectivity for catalyst coke. For example, U.S. Pat. No. 4,855,522 discloses converting C2, C3, and C4 paraffinic hydrocarbon with increased selectivity for aromatic hydrocarbon and decreased selectivity for catalyst coke. The process utilizes a dehydrocyclization catalyst comprising (a) an aluminosilicate having a silica:alumina molar ratio of at least 5 and (b) a dehydrogenation compound of (i) Ga and (ii) at least one rare earth metal. The reference discloses carrying out the aromatization conversion at a space velocity (LHSV) in the range of from 0.5 to 8 hr−1, a temperature ≥450° C. (e.g., 475° C. to 650° C.), a pressure of from 1 bar to 20 bar, and a feed contact time of 1 to 50 seconds.
More recently, U.S. Pat. No. 7,186,871 discloses that increasing the catalyst's dehydrogenation metal loading lessens the amount of catalyst coking. Although coking is lessened, increasing dehydrogenation metal loading has been found to increase the catalyst's hydrogenolysis activity, resulting in an increase in the amount of methane and other light saturated hydrocarbon in the reaction product and a decrease in the amount of the desired aromatic hydrocarbon.
There is a need, therefore, for processes which selectively convert light paraffinic hydrocarbon to C5+ hydrocarbon at high conversion with decreased yield of catalyst coke and a decreased yield of hydrogenolysis byproducts compared to conventional processes.